:root {
  --matrix-green: #00ff41;
  --matrix-green-dim: #00a82c;
  --matrix-green-faint: #044d18;
  --matrix-bg: #000000;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background-color: var(--matrix-bg);
  color: var(--matrix-green);
  font-family: "Courier New", "Consolas", monospace;
  line-height: 1.5;
  padding: 2rem 1rem;
}

/* Vertically-centered layout, used by the homepage and index pages. */
body.center {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 2rem;
  text-align: center;
}

h1 {
  margin: 0;
  font-size: clamp(2rem, 8vw, 4rem);
  letter-spacing: 0.15em;
  text-shadow:
    0 0 5px var(--matrix-green),
    0 0 15px var(--matrix-green),
    0 0 30px var(--matrix-green-dim);
}

.tagline {
  margin: 0;
  color: var(--matrix-green-dim);
  letter-spacing: 0.1em;
}

/* Stacked navigation links (homepage + architecture index). */
.links {
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
  width: 100%;
  max-width: 24rem;
}

.links a {
  display: block;
  padding: 0.85rem 1.5rem;
  border: 1px solid var(--matrix-green-dim);
  color: var(--matrix-green);
  text-decoration: none;
  letter-spacing: 0.1em;
  text-transform: lowercase;
  transition: all 0.2s ease;
}

.links a:hover,
.links a:focus {
  background-color: var(--matrix-green);
  color: var(--matrix-bg);
  border-color: var(--matrix-green);
  box-shadow:
    0 0 10px var(--matrix-green),
    0 0 25px var(--matrix-green-dim);
  outline: none;
}

/* A small "back" link for content pages. */
.back {
  color: var(--matrix-green-dim);
  text-decoration: none;
  letter-spacing: 0.1em;
}

.back:hover,
.back:focus {
  color: var(--matrix-green);
  text-shadow: 0 0 8px var(--matrix-green);
}

/* ---- Content pages (diagrams) ---- */

main.content {
  max-width: 60rem;
  margin: 0 auto;
}

main.content h1 {
  font-size: clamp(1.6rem, 6vw, 2.75rem);
  margin-bottom: 0.5rem;
}

.intro {
  color: var(--matrix-green-dim);
  max-width: 48rem;
}

/* ---- OSI layer stack ---- */

.osi {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  margin-top: 2rem;
}

.layer {
  border: 1px solid var(--matrix-green-faint);
  border-left: 4px solid var(--matrix-green-dim);
  padding: 0.9rem 1.1rem;
  transition: all 0.2s ease;
}

.layer:hover {
  border-left-color: var(--matrix-green);
  box-shadow: 0 0 12px var(--matrix-green-faint);
}

.layer-head {
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  gap: 0.75rem;
}

.layer-num {
  font-weight: bold;
  color: var(--matrix-bg);
  background-color: var(--matrix-green);
  padding: 0.05rem 0.55rem;
  letter-spacing: 0;
}

.layer-name {
  font-size: 1.2rem;
  letter-spacing: 0.08em;
  text-shadow: 0 0 8px var(--matrix-green-dim);
}

.layer-pdu {
  margin-left: auto;
  color: var(--matrix-green-dim);
  font-size: 0.85rem;
}

.layer-desc {
  margin: 0.6rem 0 0.7rem;
  color: #b8ffcb;
}

.layer-examples {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

.layer-examples span {
  border: 1px solid var(--matrix-green-faint);
  padding: 0.15rem 0.55rem;
  font-size: 0.8rem;
  color: var(--matrix-green-dim);
}

.mnemonic {
  margin-top: 2rem;
  color: var(--matrix-green-dim);
  font-size: 0.9rem;
  border-top: 1px solid var(--matrix-green-faint);
  padding-top: 1rem;
}

/* ---- OSI connection diagram (two-host model) ---- */

.osi-diagram {
  margin: 1.5rem 0 2rem;
  overflow-x: auto; /* narrow screens scroll rather than squish the labels */
}

.osi-diagram svg {
  display: block;
  margin: 0 auto;
  width: 100%;
  min-width: 560px; /* keep text legible; container scrolls below this */
  height: auto;
}

.dgm-title {
  fill: var(--matrix-green);
  font: 700 15px "Courier New", monospace;
  letter-spacing: 0.06em;
}

.dgm-box {
  fill: transparent;
  stroke: var(--matrix-green-dim);
  stroke-width: 1.5;
}

.dgm-box-physical {
  stroke: var(--matrix-green);
}

.dgm-layer {
  fill: var(--matrix-green);
  font: 600 14px "Courier New", monospace;
}

.dgm-peer {
  stroke: var(--matrix-green-faint);
  stroke-width: 1.5;
  stroke-dasharray: 6 5;
}

.dgm-peer-label {
  fill: var(--matrix-green-dim);
  font: 11px "Courier New", monospace;
}

.dgm-physical-link {
  stroke: var(--matrix-green);
  stroke-width: 3;
}

.dgm-flow {
  stroke: var(--matrix-green);
  stroke-width: 2;
  fill: none;
}

.dgm-flow-head {
  fill: var(--matrix-green);
}

.dgm-flow-label {
  fill: var(--matrix-green);
  font: 600 12px "Courier New", monospace;
  letter-spacing: 0.05em;
}

.dgm-caption {
  color: var(--matrix-green-dim);
  font-size: 0.9rem;
  max-width: 52rem;
}

.dgm-caption strong {
  color: var(--matrix-green);
}